vertical-axis rotation in east kopet dagh, ne iran, inferred from … · 2019. 11. 22. · the...

Vertical-axis rotation in East Kopet Dagh, NE Iran, inferredfrom paleomagnetic data: oroclinal bending or complex local foldingkinematics?

Jonas B. Ruh1,2 • Luis Valero1,3 • Lotfollah Aghajari4 • Elisabet Beamud5 • Gholamreza Gharabeigli4

Received: 22 June 2019 / Accepted: 10 September 2019 / Published online: 25 September 2019� Swiss Geological Society 2019

AbstractThe Kopet Dagh Mountains in NE Iran result from Cenozoic tectonic inversion of Triassic and Jurassic rifts that formed

along the southern margin of the Eurasian continental plate. The Kopet Dagh defines an arcuate orogen leading to the

suggestion that oroclinal bending took place during its formation. We performed a paleomagnetic study including seven

sampling sites of Paleocene formations around the Kalat syncline in the East Kopet Dagh to test whether this part of the

mountain belt experienced vertical-axis rotation. Paleomagnetic measurements and a reversals test indicate that parts of the

collected samples may have been partially remagnetized. Overall paleomagnetic directions of all sample sites show a mean

declination of 12.5�, which is the expected direction for stable Europe in the Paleocene and therefore negates any rotation

related to regional tectonic events. Directions calculated only from reversely polarized paleomagnetic data, however,

suggest clockwise vertical-axis rotations up to 21� since the Paleocene. Numerical modelling of a viscous multi-layer

folding mimicking the Kalat syncline stratigraphy suggests that local deviations in overprinted site-mean directions and

orientation of the maximum axes of the AMS ellipsoid may be related to complex folding kinematics, acquired after

regional vertical-axis rotation, where related viscous flow of relatively weak interlayer is represented by the sampled

Paleocene formation.

Keywords Paleomagnetism � Kopet Dagh Mountains � Numerical modelling � Vertical-axis rotation � Arabia-Eurasiacollision

1 Introduction

The Kopet Dagh mountain belt is situated in northeast Iran

and extends for over * 700 km from Afghanistan in the

East to the South Caspian Basin in the West along the

border to Turkmenistan (Fig. 1). The Kopet Dagh is an

intracontinental orogen representing the tectonic boundary

between Eurasia and the Central Iranian Block (Robert

et al. 2014). The mountain range results from tectonic

inversion of a rift basin that underwent extension during

the middle Jurassic (Kavoosi et al. 2009), potentially as a

back-arc basin to the Neotethys subduction zone (today the

Zagros mountains; Fig. 1) further to the south (Brunet et al.

2003; Zanchi et al. 2006). The initiation of uplift, i.e.

tectonic inversion, of the Kopet Dagh mountains took place

during the Cenozoic, when closure of the Neotethys led to

the formation of the Zagros orogen and the Iranian conti-

nental blocks were pushed northward together with the

Arabian plate (Golonka 2004; Lyberis and Manby 1999).

Editorial handling: S. Schmid.

Electronic supplementary material The online version of thisarticle (https://doi.org/10.1007/s00015-019-00348-z) con-tains supplementary material, which is available to autho-rized users.

& Jonas B. Ruh

[email protected]

1 Institute of Earth Science Jaume Almera, ICTJA, CSIC,

Barcelona, Spain

2 Geological Institute, ETH Zurich, Zurich, Switzerland

3 Department of Geology, Autonomous University of

Barcelona, Bellaterra, Spain

4 National Iranian Oil Company, Tehran, Iran

5 University of Barcelona, Paleomagnetic Laboratory CCiTUB

at ICTJA, CSIC, Barcelona, Spain

Swiss Journal of Geosciences (2019) 112:543–562https://doi.org/10.1007/s00015-019-00348-z(0123456789().,-volV)(0123456789().,-volV)

http://orcid.org/0000-0001-7035-1453

https://doi.org/10.1007/s00015-019-00348-z

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https://doi.org/10.1007/s00015-019-00348-z

However, the exact onset of uplift is still a matter of debate

(Robert et al. 2014). Lyberis and Manby (1999) propose

that rift inversion occurred during the late Miocene,

whereas most studies advocate that the Kopet Dagh begun

to grow already * 30 Ma ago (Berberian and King 1981;

Golonka 2004; Hollingsworth et al. 2010), similar to the

uplift of the Alborz mountains (Ballato et al. 2015; Reza-

eian et al. 2012). In either case, the * 75 km of shortening

in the Kopet Dagh (Lyberis and Manby 1999) has mainly

been accommodated by reactivation of inherited rift faults

within the basement, resulting in thrusting and fault-prop-

agation folding of the overlying strata (Robert et al. 2014;

Ruh and Verges 2018).

At present, northeastern Iran accommodates ca.

4–11 mm/a of the convergence between Arabia and Eur-

asia (Reilinger et al. 2006; Vernant et al. 2004). In the

western part of the Kopet Dagh, shortening takes place

along thrusts and minor left-lateral strike-slip faults,

whereas right-lateral strike-slip faults dominate recent

deformation in the central part of the mountain range (e.g.,

between Bojnurd and Quchan; Fig. 1), according to GPS

data and earthquake focal mechanisms (Hollingsworth

et al. 2006, 2010; Tavakoli 2007). This strike-slip motion

in the western and central part of the mountain range

reportedly initiated between * 10 Ma (Hollingsworth

et al. 2008) to * 5 Ma (Shabanian et al. 2012) and has

been related to vertical block faulting and rotation. In the

eastern Kopet Dagh, shortening occurs on major basement-

involved thrust faults (Hollingsworth et al. 2010; Robert

et al. 2014).

In contrast to local block rotation related to strike-slip

faults, regional vertical-axis rotation of the western and

eastern Kopet Dagh has been proposed to have taken place

as a result of oroclinal bending (Hollingsworth et al. 2010;

Mattei et al. 2017; 2019). Hollingsworth et al. (2010)

advocate that the Alborz and Kopet Dagh mountains have

initially been linearly E–W-trending and that oroclinal

bending started contemporaneously with the uplift of the

Fig. 1 Colored upper part shows the geological map of the Kopet

Dagh and the East Alborz (modified after Afshar 1982, 1983; Afshar

et al. 1984; Afshar Harb et al. 1986; Behrouzi et al. 1993; Bolourchi

et al. 1987; Eftekhar-Nezhad et al. 1992; Jalilian et al. 1992;

Tatavousian et al. 1993). Red circled arrows indicate local vertical-

axis rotation (Hollingsworth et al. 2010). Black rectangle shows

location of Fig. 2. Gray-scaled lower left part shows Iran and its

tectonic blocks and major sutures. Blue and orange arrows indicate

paleomagnetic directions from Mattei et al. (2017; 2019) related to

oroclinal bending of the Alborz. Black square indicates location of the

colored map above

544 J. B. Ruh et al.

Kopet Dagh range * 30 Ma ago, due to the indentation of

the South Caspian rigid block into northern Iran (Allen

et al. 2003). Paleomagnetic measurements throughout the

Alborz indicate that progressive vertical-axis rotation of an

initially linear mountain belt has formed the present-day

curvature before late-middle Miocene (Fig. 1; Cifelli et al.

2015; Mattei et al. 2017; 2019). With respect to the Kopet

Dagh, the proposed oroclinal bending would result in

clockwise vertical-axis rotation in the east and anti-clock-

wise vertical-axis rotation in the west (Hollingsworth et al.

2010).

Fig. 2 Geological map with most important structural features and stratigraphic description of the Kalat syncline. Locations of sample sites P01–

P07 are indicated as red stars. Approximate location of seismic profiles of Figs. 3, 4 and 5 and the map of Fig. 15 are shown as black outlines

Fig. 3 Seismic section (a) and its interpretation (b) of a section cross-

cutting the Kalat syncline in the west. See Fig. 2 for approximate

location. Two minor north-verging thrusts in Jurassic strata root down

into basement structures. In this and the subsequent figures: pJ pre-

Jurassic, Jk Kashafrud Formation (Middle Jurassic), Jc Chaman Bid

Formation (Upper Jurassic), Jm Mozduran Formation (Upper Juras-

sic); Shurijeh Formation (Lowermost Cretaceous), Kt Tirgan

Formation (Barremian), Ksa Sarchashmeh Formation (Aptian), Ks

Sanganeh Formation (Albian), Ka Aitamir Formation (Cenomanian),

Kad Abderaz Formation (Upper Cretaceous), Kab Abtalkh Formation

(Campanian), Kn Neyzar Formation (Campanian), Kk Kalat Forma-

tion (Campanian–Maastrichtian), PEp Pestehleigh Formation (Lower

Paleocene), PEc Chehel Kaman Formation (Upper Paleocene)

Vertical-axis rotation in East Kopet Dagh, NE Iran 545

Paleomagnetism has proven to be a viable tool to

measure vertical-axis block rotations in mountain belts

(e.g., Lowrie and Hirt 1986; Schwartz and Van der Voo

1983; Sussman et al. 2004). Whereas a wealth of paleo-

magnetic studies have been conducted to obtain vertical-

axis rotation in the Alborz mountains (Cifelli et al.

2015, 2016; Mattei et al. 2015, 2017), there is only little

data available from the Kopet Dagh range (Bazhenov

1987). Here, we present results of paleomagnetic mea-

surements from several sites around the Kalat syncline

along the northern mountain front of the East Kopet Dagh

(Figs. 1, 2) in order to test the above-mentioned proposi-

tion of vertical-axis block rotation. Paleomagnetic direc-

tions and magnetic fabric determined from anisotropy of

magnetic susceptibility (AMS) measurements are discussed

to describe the effect of three-dimensional folding kine-

matics on local vertical-axis rotation (e.g., Smith et al.

2005). Furthermore, we present a simple three-dimensional

numerical experiment to better understand the folding

kinematics, in particular local rotations in the horizontal

plane, of a doubly-plunging syncline and discuss the model

outcome with respect to the paleomagnetic data.

2 Structure and stratigraphy of the Kalatsyncline

The Kalat syncline lies in the northwestern part of the

Kopet Dagh mountains along the Iran–Turkmenistan bor-

der (Fig. 1). It is situated * 100 km east of major strike-

slip faults that crosscut the entire mountain range and is

therefore not affected by local rigid block rotation related

to those lateral motions (Fig. 1). The Kalat syncline forms

an elongated structure striking parallel to the Main Kopet

Dagh Fault, which marks the northeastern boundary of the

orogeny and represents a major inherited basement struc-

ture reactivating Paleozoic normal faults (Amurskiy 1971;

Maggi et al. 2000; Robert et al. 2014). The western part of

the syncline is flanked by two parallel striking anticlines to

the SSW and the NNE, whereas deformation gets

Fig. 4 Seismic section (a) and its interpretation (b) of a section cross-

cutting the Kalat syncline in the east. See Fig. 2 for approximate

location. North-verging rooting down to the basement cuts the section

up to Upper Cretaceous strata. A lower splay of the thrust branches

into the Lower Cretaceous that acts as a layer parallel detachment

Fig. 5 Seismic section (a) and its interpretation (b) of a section

cutting parallel to the main Kalat syncline strike. See Fig. 2 for

approximate location. Several basement-related normal faults appear

in pre-Jurassic rocks. ESE-verging thrust rooting down into a

basement normal fault. Minor offsets occur in Upper Cretaceous

strata


partitioned into laterally overlapping folds forming relay

structures to the east (Fig. 2). The main fold axis of the

Kalat syncline plunges inward at very shallow angles (see

Table 1) resulting in a slightly conical appearance.

The stratigraphic column cropping out around the Kalat

syncline includes Upper Jurassic to Paleocene rocks,

excluding the Quaternary cover (Fig. 2). The following

lithological descriptions are a summary of the more

detailed investigation of Robert et al. (2014) and refer-

ences therein. The oldest rocks in the area are formed by

Late Jurassic alterations of shale and marly limestones

(Chaman Bid Formation) and well-bedded limestone to

siliciclastic deposits (Mozduran Formation) and appear

within the hinge of the anticline towards the SW. Towards

the north, and the center of the Kalat syncline, the

stratigraphy is entirely conserved. Jurassic rocks are

overlain by nine Cretaceous formations, with the lowest

one consisting of red siliciclastic sandstones and gypsum/

anhydrite deposits (Surijeh Formation) followed by marls

and orbitolinid-rich limestones (Tirgan Formation). The

upper part of the Lower Cretaceous consists of gray marls,

shale and limestones (Sarcheshmeh Formation) and inter-

layered marls and sandstones (Sanganeh Formation). The

Upper Cretaceous begins with mudstones and minor

(partially glauconitic) sandstones (Aitamir Formation)

overlain by calcareous and marly shales (Abderaz For-

mation) and calcareous shales, limestones and small rudist-

patch reefs (Abtalkh Formation). The uppermost Creta-

ceous consists of glauconite deposits, clays and minor

sandstones (Neyzar Formation) progressively transitioning

into bioclastic limestone and carbonate build-ups (Kalat

Formation). Paleocene deposits crop out in the hinge of the

Kalat syncline and consist of red continental silt- and

sandstones and minor conglomerates (Pestehleigh Forma-

tion) and overlying white carbonates (Chehel Kaman

Formation). The stratigraphic succession indicates shallow

marine to continental environments with two main hiatuses

and local erosion located at the top of the Aitamir and

Kalat formations.

In the following paragraphs, the deeper structure of the

Kalat syncline is described by interpreting three seismic

sections (Fig. 3, 4, 5). Seismic data has been processed at

the National Iranian Oil Company (NIOC). Their inter-

pretation is based on surface geology, boreholes and cross-

correlation with other seismic profiles from the region.

Two sections crosscut the fold in a perpendicular manner

from SW to NE and one along strike from WNW to ESE

(see Fig. 2 for location). All seismic sections reach down

to undifferentiated pre-Jurassic basement (pJ) and its

contact to the overlaying Middle Jurassic Kashafrud For-

mation (Jk), which was deposited in a rift system (see also

Robert et al. 2014) succeeding the Cimmerian orogeny

(Wilmsen et al. 2009). Seismic data of the western section Table1

Paleomagnetic

datafrom

theKalat

syncline

Location

BTC

ATC

Site

Latitude(�N)

Longitude(�E)

Form

ation

Lithology

Bedding

N/n

D(�)

I(�)

k(-

)a95(�)

D(�)

I(�)

k(-

)a9

5(�)

KD17-01

36�550 49.1100

59�550 39.9000

Pestehleigh

Red

siltstone

258/03

14/7

29.6

36.3

6.06

26.7

27.9

38.2

6.06

26.7

KD17-02

36�580 48.8700

59�570 9.2200

Pestehleigh

Red

siltstone

014/14

16/9

11.4

52.9

5.38

24.5

12

38.9

5.39

24.5

KD17-03

37�0

0 0.0100

59�430 27.1300

Pestehleigh

Red

siltstone

050/17

13/11

347.4

53.6

21.64

10

2.4

44.4

21.9

10

KD17-04

37�1

0 44.1200

59�440 14.7600

Pestehleigh

Red

siltstone

085/06

9/4

14

49.6

10.83

29.3

20.3

47.3

10.82

29.3

KD17-05

36�590 41.5300

59�460 51.6800

Pestehleigh

Red

siltstone

030/27

14/13

2.8

51.1

32.11

7.4

11.4

26.1

32.05

7.4

KD17-06

37�0

0 57.7200

59�480 39.3400

Pestehleigh

Red

siltstone

216/02

11/2

329

58.3

57.54

33.5

325.9

59

57.66

33.5

KD17-07

37�3

0 2.3400

59�450 45.9400

Chehel

Kam

anCarbonate

210/29

11/2

8.7

34.4

6.03

135.8

353

60.1

6.05

135.3


and its interpretation illustrate an open syncline with a

wavelength of * 15 km (Fig. 3). Small rift-related faults

are observed within the pre-Jurassic basement reaching

into the syn-rift Jurassic Kashafrud Formation (Fig. 3). The

small offset shown by those faults and the smooth contact

between Middle to Upper Jurassic strata indicates little to

no fault reactivation during Cenozoic shortening. However,

several thrusts can be interpreted within the folded strata,

all verging towards the NE (Fig. 3). The most prominent

are cutting the Middle Jurassic in the SW resulting in a

minor fault-bend fold and two fish-tail structures in the

Cretaceous strata, probably branching into potential

detachment horizons, as suggested by Robert et al. (2014).

In the east of the syncline, the interpretation of

a * 14 km long seismic profile suggests major implication

of northeast-verging thrust faults (Fig. 4). Fault-related

deformation includes a shallow angle thrust fault cutting

through most parts of the stratigraphic sequence, including

the pre-Jurassic basement, branching into a Lower Creta-

ceous layer-parallel detachment. A splay of this fault

crosscuts the Lower Cretaceous strata and branches into

Upper Cretaceous formations (Fig. 4). Furthermore,

thrusting of the pre-Jurassic units and related fault-propa-

gation folding of the overlaying Jurassic strata indicate

normal fault reactivation within the basement. On the

surface, the resulting structure appears as two synclines,

with hinges at * 1 km and * 10 km in the seismic sec-

tion, and a central anticline at * 6 km (Fig. 4), as illus-

trated in the geological map (Fig. 2).

The seismic profile parallel to the main strike of the

orogen cuts the Kalat syncline along * 35 km direction

WNW–ESE (Fig. 5). The interpretation of this seismic

section illustrates an open syncline with shallow-dipping

limbs. The contact of the Kalat and the Pestehleigh for-

mations plunges inward at an angle\ 5�, taken into

account the vertical exaggeration. Some compressional

structures such as an east-verging thrust cutting Jurassic

and Lower Cretaceous strata in the east and a fish-tail

structure and some small thrusts in the Cretaceous in the

west are observed (Fig. 5). A steeply-dipping offset within

the pre-Jurassic basement forming a typical half-graben

suggests the occurrence of N–S trending rift structures

below the Kalat syncline (e.g., Brunet et al. 2017; Khain

et al. 1991). The hinge zone of the Kalat syncline is located

above the basement-related half-graben structure, where

the contact between pre-Jurassic to Jurassic units is deepest

(Fig. 5). However, whether basement faults have been

reactivated during the Cenozoic growth of the mountain

range cannot be confirmed from the seismic profile.

Potentially, inherited normal faults triggered localization of

deformation during folding.

3 Methodology

3.1 Sampling

In order to obtain vertical-axis rotation of the Kalat syn-

cline related to the uplift of the Kopet Dagh, paleomagnetic

samples have to be collected from stratigraphic units older

than Oligocene, the earliest suggested timing for the initi-

ation of shortening in NE Iran (Hollingsworth et al. 2010).

We collected a total of 93 sample cores from six sites

situated in the Paleocene Pestehleigh Formation, which

contains reddish continental sediments, and one site from

the overlaying Chehel Kaman Formation, which consists of

white carbonates (Fig. 2; Table 1). In the Pestehleigh

Formation, sampling was restricted to the fine-grained

components, i.e. clays and siltstones. Sampling sites have

been chosen to cover different parts of the syncline,

including both limbs and the hinge zone, the western and

the eastern part (Fig. 2).

3.2 Magnetic measurements

Initially, the natural remanent magnetization (NRM) of

every sample was measured before stepwise demagneti-

zation due to heating or an induced alternating field (AF).

NRM and magnetization were measured after every

demagnetization step using a 2G Enterprise 755R, three-

axis DC superconducting quantum interference device

(SQUID) rock magnetometer (background magnetic

moment around 10-9 emu/cc) at the Paleomagnetic Lab-

oratory at the ICTJA–CSIC in Barcelona. One specimen of

every core (93 samples) has been stepwise demagnetized

thermally in 18 steps up to a maximum of 670 �C. Samples

were both heated and cooled for at least 45–60 min,

respectively, with an ASC TD48 oven with an internal field

of less than 10 nT. Additionally, AF demagnetization was

applied to 35 samples using a ASC D-Tech 2000 AF

demagnetizer. AF demagnetization was obtained by 18

steps up to an alternating field of 200 mT. Furthermore, 10

of the AF demagnetized samples were further thermally

demagnetized until complete loss of magnetization.

Demagnetization curves were illustrated with vector end

point diagrams to obtain different magnetic components

depending on temperature or AF intensity (Zijderveld

1967). The orientation of different magnetization compo-

nents was identified by using the Remasoft software

(Chadima and Hrouda 2006), where line fitting of principal

components is achieved with the method of Kirschvink

(1980) and mean directions of the characteristic compo-

nents at site level are calculated by the method of Fisher

(1953).


A cross-component isothermal remanent magnetization

(IRM) was induced on three samples (P01b, P05c, P07e)

and thermally demagnetized to identify the mineralogy of

the magnetic carrier by relating unblocking temperatures

and coercivities of specific ferromagnetic minerals (Lowrie

1990). IRM was induced by a ASC impulse magnetizer

IM10-30 along three orthogonal directions in successively

smaller fields of 1.2 T (z-axis), 0.3 T (y-axis) and 0.1 T (x-

axis). Thermal demagnetization was carried out with a

Schonstedt TSD-1 thermal demagnetizer and the intensity

was measured with a spinner magnetometer JR6A (Agico).

3.3 AMS measurements

Anisotropy of low-field magnetic susceptibility (AMS)

measurements allows recognizing the average preferred

orientation of particles within a sample and can be used to

infer tectonic fabric in weakly deformed rocks (Borradaile

and Henry 1997). The orientation and values of the three

principle axes of the AMS ellipsoid are achieved by mea-

suring the susceptibility, i.e. the potential of magnetization

in response to an applied field, along 15 different directions

of the sample (Jelinek 1981). The shape of the AMS

ellipsoid results from both magnetic and physical proper-

ties of all grains within a rock sample (Hrouda 1982;

Uyeda et al. 1963) and indicates whether the magnetic

fabric is sedimentary or overprinted due to layer parallel

shortening (Weil and Yonkee 2009; Fig. 6): During sedi-

ment accumulation, the minimum axis of the AMS ellip-

soid is vertically oriented with the maximum and

intermediate AMS principal axes randomly distributed in

the horizontal plane (stage A). Initial tectonic overprint is

characterized by the maximum principal axis aligning

perpendicular to the shortening direction (stage B). With

further shortening, the minimal principal axis of the AMS

ellipsoid rotates in a vertical plane (stage C) towards being

parallel with the shortening direction (stage D). The fully

overprinted tectonic fabric of the AMS ellipsoid shows a

minimum axis parallel to shortening and a vertically ori-

ented maximum axis (stage E). AMS of each sample was

measured before demagnetization with an AGICO Kap-

pabridge KLY-2 susceptibility bridge with an operating

frequency of 920 Hz and a sensitivity of 4.10-08 for a

specimen of nominal volume of 10 cc. AMS results are

illustrated with the Anisoft software (Chadima et al. 2018).

4 Rock-magnetic results

The majority of measured samples show intensities of the

NRM between 2 9 10-4 and 3 9 10-2 A/m (Fig. 7).

White carbonate samples from the Chehel Kaman Forma-

tion (site P07) exhibit relatively low intensities in contrast

to samples from the red beds of the Pestehleigh Formation

(sites P01–06). Values of initial magnetic susceptibility

(before thermal or AF demagnetization) corrected for

sample volume range between 3 9 10-5 and 3 9 10-4

[SI] (Fig. 7). Similar to intensity, carbonate samples show

the lowest observed susceptibilities. The ratio of NRM

intensity to magnetic susceptibility (normalized intensity in

A/m) can provide a first-order indication of magnetic

mineralogy variations. Most measured samples plot within

a normalized intensity range of 5–20 A/m (Fig. 7), sug-

gesting that there is no major change in magnetic carrier

for the different sample sites.

Three-component IRM demagnetization measurements

on three selected samples have been conducted allowing

for a better interpretation of the ferromagnetic mineral

content (Fig. 8; Lowrie 1990). In general, the thermally

distributed intensity decay observed in all samples may

obey to a mixture of remanence carriers with a wide range

of grain-sizes. Thermal demagnetization curves of the soft

(\ 0.1 T), medium (0.1–0.3 T) and hard (0.3–1.2 T)

coercivity fractions of sample P01b show similar

Fig. 6 Schematic illustration of the evolution of the AMS ellipsoid

(equal-area lower hemisphere projections) in response to layer

parallel shortening (after Saint-Bezar et al. 2002; Weil and Yonkee

2009). Stage A: Purely sedimentary fabric; stages B and C:

Composite sedimentary-tectonic fabric; stages D and E: Tectonic

fabric. Black arrows indicate shortening direction related to the lower

hemisphere plots


proportion of the different coercivity fractions with distinct

unblocking curves (Fig. 8a) Soft fraction shows a minor

change in slope at 350 �C, suggesting certain contents of

low-coercivity maghemite or titanomagnetite. The main

decay of the soft component occurs at 500–560 �C, withalmost no magnetization left in the interval 560–650 �C,indicating low-coercivity magnetite (i.e., MD magnetite).

The medium and hard fractions of the IRM decrease

monotonically up to 620 �C and demagnetize completely at

650 �C, pointing to hematite with some Ti content.

Individual demagnetization curves of cross-component

IRM of sample P05c indicate similar ferromagnetic carriers

although in different proportions (Fig. 8b): Magnetization

is mainly carried by hard coercivity minerals, followed by

low coercivity minerals and negligible intermediate coer-

civity fraction. Minor changes in demagnetization curve

slope of the soft fraction at 150 �C and 350 �C point

toward titanomagnetite and low-coercivity maghemite as

potential carriers. Abrupt and complete demagnetization at

530–560 �C indicates magnetite. The medium fraction is

initially rough but generally decreases linearly towards

640 �C indicating hematite with coercivities between 0.1

and 0.3 T. The hard fraction decreases steeply until 150 �C,pointing towards goethite. The high temperature fraction

demagnetizing completely at 650 �C indicates hematite.

Demagnetization curves of sample P07e demonstrate

that magnetite is the main carrier in the carbonates of this

site (Fig. 8c): All individual curves almost entirely

demagnetize at 560 �C with the soft fraction being the most

intense, potentially indicating the dominance of multi

domain magnetite. Hematite only contributes * 1% of the

total IRM. Minor changes in demagnetization curve slopes

of the soft and medium fractions point toward titanomag-

netite and low-coercivity pyrrhotite or maghemite.

Only sample P05c showed a distinctive decrease of

intensity of the hard fraction at very low temperatures

(80–120 �C) suggesting the presence of goethite (Fig. 8b).

However, goethite often only contributes to IRM as a result

to an induced field larger than 1.5 T (Lowrie and Heller

1982). This means that occurrences of goethite in tested

samples might not be represented by the cross-component

IRM demagnetization curves. Nevertheless, goethite may

still be detectable in the NRM demagnetization curves due

to its very low Curie and Neel temperatures.

Fig. 7 Bulk susceptibility versus magnetic intensity plot for all

sampling sites. Color code is given in the legend. Dashed lines denote

intensity normalized for susceptibility, which serves as a first-order

indicator for variations of magnetic carrier

Fig. 8 Cross-component IRM thermal demagnetization curves for

three selected samples. Soft, medium and hard coercivity fractions

relate to 0.1, 0.3 and 1.2 T, respectively. a, b Samples P01b and P05c

(Pestehleigh Formation: continental red beds) show a mixture of low-

coercivity magnetite and hematite with a large range of coercivities.

c Sample P07e (Chehel Kaman: white carbonates) indicates low- to

medium-coercivity magnetite as magnetic carrier


In general, a clear occurrence of magnetite and hematite

is detected, whereas the presence of pyrrhotite, maghemite

and goethite is speculative due to a lack of indicative Curie

temperatures for these phases.

4.1 Paleomagnetic directions

Figure 9 illustrates three typical demagnetization curves of

the NRM for samples P02b, P03 g and P05d including

Zijderveld plots and normalized intensity. Most demagne-

tized samples reveal a low-temperature or low-AF com-

ponent, respectively. Pure thermal demagnetization leads

to loss of * 50% of the NRM at around 200 �C (Fig. 9a).

Further demagnetization reveals a single static component

of magnetization that gets completely demagnetized at

620 �C. AF demagnetization of the NRM results in a

severe loss of 40–60% of the magnetization at alternating

fields of\ 20 mT (Fig. 9b, c). Sample P02b reveals further

demagnetization above 100 mT with a remaining magnetic

intensity of 10% at an alternating field of 200 mT (Fig. 9b).

Further thermal demagnetization of the same sample up to

550 �C unblocks a high-coercivity component with a

reverse orientation above 400 �C heating. The demagneti-

zation curve of sample P05d shows that * 50% of the

magnetization is related to a high-coercivity component

that could not be removed by AF demagnetization

Fig. 9 Representative demagnetization curves (tilt corrected) and

related intensity plots. Blue and green points represent the vertical

and horizontal projections of the magnetic direction (after Zijderveld

1967). Full circles indicate thermally demagnetized steps, empty

circles denote AF demagnetization. a Pure thermal demagnetization

curve of sample P03 g resulting in normal ChRM. b Mixed AF and

thermal demagnetization curve of sample P02b showing reverse

ChRM. c Mixed demagnetization showing normal ChRM of sample

P05d


(Fig. 9c). Magnetic intensity drops to 25% of its initial

value by further heating to 80 �C, indicating the presence

of goethite or viscous magnetization. A single static normal

magnetic component is then revealed by stepwise heating

up to 620 �C (Fig. 9c). In all samples, characteristic

remanent magnetization (ChRM) has been defined as the

highest-temperature component observed.

A distinct low-temperature or low-AF component has

been detected in 43 samples (Fig. 10). Correcting the

component directions for bedding tilt results in clustering

around a mean declination of 3.6� and a mean inclination

of 43.5� with a Fisher precision parameter of k = 13.64 and

a radius of circle of confidence of a95 = 6.1�. Uncorrecteddirections cluster with statistical parameters of k = 22.31

and a95 = 4.7� and average along an orientation with a

declination of 356.7� and an inclination of 53.7� (Fig. 10).These data suggest that the low-temperature, low-AF

components developed after tectonic folding of the Kalat

syncline and are a result of the recent Earth’s magnetic

field that exhibits an inclination of 53� for a latitude of

37�N and a longitude of 60�E (http://www.ngdc.noaa.gov/

geomag-web/#igrfwmm).

A well-characterized ChRM component has been found

in * 50% (48/93) of demagnetized samples (Table 1).

From the ChRM directions, the mean paleomagnetic

direction for every site has been obtained by using Fisher

(1953) statistics (Fig. 11a–g). The ChRM mean direction

for every site has been calculated with reversed directions

transposed to normal. Exact location, lithology, mean ori-

entation and according statistical parameters of every site

are listed in Table 1. Sites P06 and P07 only exhibit two

ChRM directions and were therefore excluded from further

consideration (Fig. 11f and g). Declination and inclination

of mean directions of transposed ChRM of sample sites

P01–P05 range between 2.4–27.9� and 26.1–47.3�,respectively (Fig. 11a–e). The overall mean of all relevant

site-mean directions (P01–P05) is oriented along a decli-

nation of 12.5� with an inclination of 37.3�, where k = 10.2

and a95 = 7.1� (Fig. 12a). All measured directions are

separated into normal and reverse oriented partitions to test

the primary origin of the ChRM (Fig. 12b, c). Tilt-cor-

rected normal directions show a mean orientation with a

declination of 4.8� and an inclination of 38� (Fig. 12b).

The tilt-corrected reverse ChRM mean in transposed form

has a declination of 33.5� and an inclination of 37.5�(Fig. 12c). Accordingly, a reversal test including sites P01–

P05 was performed resulting in an angle of c = 22�between the mean directions of the normal and reverse

polarities (McFadden and McElhinny, 1990). The obtained

angle c is larger than the critical angle cc = 20� indicating anegative reversal test for the two separate clusters of ori-

entations. This identifies potential magnetic component

overlap (overprint) of samples collected from the clastic

Pestheleigh Formation.

4.2 Magnetic fabric

Before demagnetization, AMS ellipsoids have been

obtained for samples from every site (Fig. 13; Table S1 in

supplementary materials). The resulting AMS ellipsoids

then can be compared to conceptual models for the evo-

lution of AMS fabric as a result of horizontal shortening to

infer local strain/stress patterns (Fig. 6). Measured AMS

ellipsoids of sample sites P01–P07 show a range of dif-

ferent patterns from sedimentary to tectonically over-

printed fabrics. P02 and P05 show maximum AMS axis

scattered in the horizontal plane, although statistically

clustering along 90� and 300�, respectively, with a roughly

vertical minimum AMS axis (Fig. 13b, e). These AMS

patterns describe the initial transition from purely sedi-

mentary to composite sedimentary-tectonic fabric (stage A

to stage B; Fig. 6). Sites P01, P03 and P04 are

Fig. 10 Equal-area lower hemisphere projection of the low-temper-

ature/AF component of 43 samples. Top: Magnetic directions

corrected for bedding tilt. Bottom: No tectonic correction. Pink circle

indicates mean direction. Dec declination, Inc inclination, k precision

parameter, a95 confidence angle


http://www.ngdc.noaa.gov/geomag-web/#igrfwmm

http://www.ngdc.noaa.gov/geomag-web/#igrfwmm

characterized by clear alignment of the maximum AMS

axis in the horizontal plane with a vertical minimum axis,

scattering slightly along a vertical plane normal to the

maximum axis (Fig. 13a, c, d). These patterns represent

composite sedimentary-tectonic fabric (stage B and C;

Fig. 6). Finally, sites P06 and P07 illustrate alignment of

maximum and minimum AMS ellipsoid axis both in the

horizontal plane (Fig. 13f, g), where site P07 shows com-

posite sedimentary-tectonic fabric between stage C and D

(Fig. 6) and P06 indicating tectonic fabric overprint (stage

D to E; Fig. 6).

The corrected degree of anisotropy P (parameters are

defined in Table S1) is higher for carbonate samples of site

P07, indicating that those high values are controlled by

rock composition (Fig. 13h). Other than P07, P does not

show any distinctive correlation for different sites, sug-

gesting that it is mainly strain controlled or dependent on

the bulk susceptibility (see Table S1 in supplementary

materials). Calculated values of the shape parameter

T (Jelinek 1981) scatter from - 1 (prolate) to 1 (oblate)

with only site P06 showing purely oblate (tectonic) shapes

of AMS ellipsoids (Fig. 13i). Conducted f and elongation

Fig. 11 a–g ChRM directions of each site (P01–P07) projected onto

an equal-area lower hemisphere. Pink circles: Mean direction and

confidence circle of transposed magnetic directions. Pink circles:

Overall mean and confidence circle of all site-mean directions. Dec

declination, Inc inclination, k precision parameter, a95 confidence

angle


tests on all specimens demonstrate that results are statisti-

cally significant, except for specimens of site P07, which

show very low f values (\ 3.48) and a large e31 angle

([ 26.5�; see Table S1).

5 Numerical model

Additional to the magnetic measurements, we conducted a

simple numerical experiment of three-dimensional multi-

layer folding in order to understand the potential effect of

folding dynamics on local vertical-axis rotation. It is

assumed that the clastic, silt-dominated Pestehleigh For-

mation represents a relatively weak stratal layer in between

mechanically stronger carbonate lithologies, that are the

Cretaceous Kalat Formation below and the Paleogene

Chehel Kaman Formation above. Viscous flow and verti-

cal-axis rotation within the mechanically weak layer are

tracked during folding of the multi-layer experiment. The

obtained results can then be compared to the paleomagnetic

measurements to discuss whether the observed paleomag-

netic rotations and AMS data fit the deformation and stress

field predicted by the numerical experiment.

5.1 Mathematical description and model setup

The experiment is carried out with a three-dimensional

numerical code based on the finite different method with a

fully-staggered Eularian grid and freely advecting

Lagrangian markers carrying rock information (I3ELVIS;

Gerya 2010; Gerya and Yuen 2007). The numerical model

solves the equations for conservation of mass and

momentum (Stokes equation) with an efficient OpenMP-

parallelized multigrid solver applying a simple linear vis-

cous rheology.

The experiment has been set up to reproduce an elon-

gated, doubly-plunging open syncline in size and shape

comparable to the Kalat syncline (Figs. 2, 3, 4 and 5). The

Eulerian model box measures 30�14.8�58.8 km in x-, y- and

z-direction (Fig. 14a) with a nodal resolution of

101�149�197, respectively, and 8 Lagrangian markers per

nodal cell. To investigate the potential viscous flow in the

Paleocene Pestehleigh Formation, a multilayer geometrical

setup has been introduced that represents the uppermost

crustal sequence outcropping along the Kalat syncline

(Fig. 2). Initial layering of the experiment includes two

strong horizons with a viscosity o 1023 Pa�s separated and

embedded within a weak matrix with a viscosity of 1020

Pa�s (Fig. 14a). Both strong layers have an initial vertical

thickness of 2 km and are introduced at y = 7.3–9.3 km

and y = 10.3–12.3 km. The strong layers represent the

rigid Cretaceous carbonate formation below (Kalat For-

mation) and the Paleogene white carbonates above (Chehel

Kaman Formation) the mechanically weaker red clays and

siltstones of the Paleocene Pestehleigh Formation.

To obtain a doubly-plunging syncline, layers have been

shortened horizontally in two perpendicular directions.

Velocity boundary conditions prescribe material influx

along all lateral boundaries resulting in a total shortening

rate of 1 cm/year along the x-axis and a total shortening

Fig. 12 Overall statistics of site measurements in geographic and tilt-

corrected coordinates (Fig. 11a). a Transposed mean directions of

sample sites P01–P05. b Normal oriented measurements of sites P02–

P05. c Reverse oriented measurements of sites P01 m, P02 and P04.

Pink circles: Overall mean and confidence circle of all site-mean

directions. Dec declination, Inc inclination, k precision parameter, a95confidence angle


rate of 0.5 cm/year along the z-axis (Fig. 14a). The top

velocity boundary prescribes material outflow by a vertical

velocity of –0.62 cm/yr to assure conservation of volume

(mass) within the Eulerian model box. The bottom

boundary prescribes zero perpendicular material flux. All

boundaries exhibit free-slip velocity conditions in both

boundary-parallel directions, which means zero shear

stresses across the boundaries. The numerical experiment

run with timesteps of 5 kyr.

5.2 Modelling results

Two-axial horizontal shortening of the multilayer stack

described above for a time span of 1.35 Myr resulted in

parallel folding of the strong layers and viscous flow due to

bedding-parallel shearing of the weak layer in between.

Removal of the uppermost weak layer allows recognizing

the fold geometry, defining an open doubly-plunging syn-

cline (Fig. 14a). Fold wavelengths in x- and z-directions

Fig. 13 a–g Equal-area lower hemisphere projection of the AMS

ellipsoids for each sample site (P01–P07). Blue squares: maximum

susceptibility axis (K1); Green triangles: intermediate susceptibility

axis (K2); Pink circles: minimum susceptibility axis (K3). Red lines:

Bedding planes. Specimens of all sites are statistically significant

except for P07 (see Table S1). h Mean susceptibility versus degree of

corrected anisotropy (P) plot. All sample sites show similar degree of

anisotropy except carbonate samples of site P07. i Plot of degree of

corrected anisotropy (P) versus shape factor (T). All sites show both

oblate and prolate shapes of magnetic fabric except site P06 (only

oblate) indicating full tectonic overprint


are comparable to strike-orthogonal (NNE–SSW) and

along-strike (WNW–ESE) folding wavelengths of the

Kalat syncline, respectively (Fig. 2, 3, 4 and 5). Figure 14b

illustrates the horizontal shear strain rate (total vertical-axis

rotation) and principal stress directions within the

interbedded weak horizon in a plane cutting the model box

horizontally at y = 7.4 km (Fig. 14a). Vertical-axis shear

strain rate is defined as

_exz ¼ _ezx ¼1

2

omxoz

þ omzox

� �; ð1Þ

where vx and vz are velocities in x- and z-direction and

x and z denote spatial coordinates.

Vertical-axis rotational shear strain rates within the

weak horizon representing the Pestehleigh Formation show

two distinctive patterns depending on their stratigraphic

position. At the top of the weak layer, close to the contact

with the upper strong layer, viscous flow shows clockwise

rotational shear strain rates in the lower left and the upper

right quarter and counter-clockwise rotation in the upper

left and lower right quarter (Fig. 14b). A contrary

scheme of vertical-axis rotation occurs at the base of the

weak horizon, along the contact to the underlying strong

layer. There, viscous flow undergoes clockwise rotation in

the upper left and lower right quarters and counter-clock-

wise rotation in the lower left and upper right parts,

respectively (Fig. 14b).

Principle stress within the horizontal plane cross-cutting

at y = 7.4 km show an orientation parallel to the x-axis

(main shortening direction) where shear strain rates are

close to zero, which is close to the stratigraphic center

(Fig. 14b). Towards the strong layers, principle stress axes

get deflected depending on the viscous vertical-axis rota-

tion within the weak horizon. A typical pattern of stress

orientation can be detected in the right part of the hori-

zontal slide (Fig. 14b): (i) Along the contact with the upper

strong layer, maximum principle stress axes tend to deflect

inwards directing towards the syncline center. (ii) Close to

the contact with the lower strong layer, maximum principle

stresses rotate towards parallel to the contact, away from

the syncline center.

6 Discussion

In the following section, we first discuss the applicability of

the presented paleomagnetic data. Then, measured paleo-

magnetic directions are discussed with regard to potential

vertical-axis rotation in the East Kopet Dagh mountains

(e.g., Hollingsworth et al. 2010; Mattei et al. 2019) and the

Alborz (e.g., Cifelli et al. 2015; Mattei et al. 2017). Finally,

we compare paleomagnetic results with the obtained 3D

numerical experiment to evaluate the effects of folding-

related flow on local vertical-axis rotation and principle

stress directions.

6.1 Quality of the paleomagnetic data

Cross-component IRM measurements showed that med-

ium- to high-coercivity hematite is the main carrier of

magnetic components in the continental Pestehleigh For-

mation (Fig. 8a, b). Hematite has high Neel temperature

Fig. 14 Numerical model of viscous multi-layer folding. a 3D

illustration after 1.35 Myr of shortening. Grey: Weak layers with a

viscosity of g1 = 1020 Pa�s. Black: Strong layers with a viscosity of

g2 = 1023 Pa�s. Topography of the top of the upper strong layer

presents a doubly-plunging syncline. Botton right: Map view of

model box with applied velocity boundary conditions. White line

indicates location of horizontal slice shown in b. b Colored area

indicates shear strain rate within intermediate weak layer. Blue

indicates clockwise vertical-axis rotation. Red denotes counter-

clockwise vertical-axis rotation. Black lines show the orientation of

the maximum principal stress axis. The shear strain rate and stress

pattern strongly depend on the stratigraphic position of the weak

intermediate horizon


and saturates at high magnetic fields (^ 675 �C, 1.5–5 T;

Lowrie 1990 and references therein) and therefore serves

as good potential carrier of stable ChRM components.

Continental red bed sandstones can be prone to hematite-

related remagnetization due to dissolution of iron-bearing

silicates (Hodych et al. 1985; Jiang et al. 2017). Therefore,

we focused on sampling fine-grained clay and siltstones to

avoid chemical magnetic overprint. The presence of goe-

thite is not demonstrated clearly by cross-component IRM

demagnetization curves but can be inferred from combined

temperature-AF demagnetization curves of the NRM

(Fig. 9b, c). However, the effect of (potentially secondary)

goethite on the ChRM component is removed efficiently

due to its low Curie and Neel temperatures of * 120 �C(Ozdemir and Dunlop 1996). In general, magnetic miner-

alogy of sample sites P01–P06 (Pestehleigh Formation) is

similar to lithologies sampled to reveal vertical-axis rota-

tion in the Alborz (Mattei et al. 2017).

Cross-component IRM demagnetization of carbonate

samples from site P07 mainly indicate magnetite with low

coercivities as magnetic carrier (Fig. 8c). Furthermore,

samples of site P07 exhibit relatively low NRM intensities

and bulk susceptibilities in contrast to the continental

sediments of site P01–P06 (Fig. 7). Independent of the

quality of the magnetic data, site P06 and P07 only com-

prise two samples each that reveal a paleomagnetic direc-

tion due to a stable ChRM component (Fig. 11f, g). Thus,

these two sites have not been considered when calculating

the overall paleomagnetic directions (Fig. 12) and their

resulting site-specific paleomagnetic rotation is ignored

during further discussion. For sites P01–P05, two-thirds of

all measured samples revealed a stable ChRM component

(Table 1). A fold test was not possible to conduct given the

very shallow dips of bedding within the Pestehleigh For-

mation around the Kalat syncline. Nevertheless, the

occurrence of normal and reverse polarities at site level in

sites P01, P02 and P04 reinforces the pre-tectonic character

of the collected data (Fig. 12b, c). The applied reversal test

(McFadden and McElhinny 1990) is negative (see

Sect. 4.1), indicating at least partial magnetic overprint

(Fig. 12b, c). Therefore, ChRM orientations are discussed

in the following with respect to potential magnetic over-

print or at least partial incomplete isolation of the ChRM

directions.

Ellipsoids of the anisotropy of the magnetic suscepti-

bility (AMS) show good clustering for each sample site

except for sites P02 and P05, where the clusters of the

different ellipsoid axes are more dispersed (Fig. 13). All

sites show a certain amount of tectonic overprint of the

magnetic fabric with the minimum AMS ellipsoid axis

parallel to the strike of the mountain belt and the maximum

AMS ellipsoid axis oriented into the direction of horizontal

shortening (e.g., Weil and Yonkee 2009).

6.2 Vertical-axis rotation of the East Kopet Dagh

The Kopet Dagh mountain belt resulted from Oligocene–

Miocene tectonic inversion of a mainly Jurassic intracon-

tinental rift situated on the Turan plate, along the southern

margin of the Eurasia continent (Golonka 2004; Lyberis

and Manby 1999). In this study, we gathered paleomag-

netic directions in the East Kopet Dagh to test whether

shortening and uplift in this part of the orogen was

accompanied by vertical-axis rotation, as inferred from its

arcuate shape (Fig. 1) and as proposed by earlier studies

(e.g., Hollingsworth et al. 2010). To calculate vertical-axis

rotation, paleomagnetic directions presented here have to

be compared to Paleocene paleomagnetic directions from

stable southern Eurasia. Cretaceous to Paleocene rocks in

the (today) Turkmen part of the northwestern Kopet Dagh

revealed a paleomagnetic direction with a declination of

12.2� and an inclination of 49.7� (Bazhenov 1987). This

direction agrees with the paleomagnetic direction calcu-

lated from the mean paleomagnetic pole for stable Europe

with a declination of 12.5� and an inclination of 50.1� forthe Paleogene (Gordon and Van der Voo 1995). The

overall mean paleomagnetic direction (normal and reverse

orientations) obtained from Paleocene red beds of the Kalat

syncline in the East Kopet Dagh shows a declination of

12.5� with an inclination of 37.3� (Fig. 12a). Hence, this

data would suggest that the Kalat synclinal structure has

not been prone to vertical-axis rotation other than the

rotation related to the stable Eurasian continental plate

since Paleocene times. The relatively flat inclination in

contrast to the expected stable Eurasia paleomagnetic

inclination might be due to inclination shallowing typically

observed in continental red bed formations (Garces et al.

1996; Tan and Kodama 2002). However, an applied

reversal test indicating that ChRM components are par-

tially overprinted, which makes this interpretation unreli-

able, as magnetic directions may be overprinted after

tectonic rotation. Taking into consideration only the

reverse ChRM directions, presumably not overprinted, the

resulting transposed mean direction exhibits a declination

of 33.5� with an inclination of 37.5� (Fig. 12c). With

respect to paleopoles for stable Europe with a declination

of 12.5�, this would indicate a clockwise vertical-axis

rotation of 21�. However, the reversed directions show a

wide clustering and a a95-angle of 17.3�.We interpret that at least the reversed directions have

been acquired shortly after Paleocene sediment accumu-

lation and have remained stable since then, whereas parts

of the normal directions have been overprinted due to

tectonic activity, resulting in a negative reversal test. This

would imply that the reverse orientations can be used to

calculate vertical-axis rotations since the Paleocene


(depositional age of the Pestehleigh Formation) and that

the normal orientations are the result of magnetic overprint

having taken place after the main phase of regional

deformation. However, variations in mean transposed

normal directions may indicate late stage (after magnetic

overprint) tectonic activity.

Paleomagnetic data from the Alborz mountains suggest

syn-orogenic oroclinal bending of the range as a result of

the indentation of the rigid South Caspian Basin (Mattei

et al. 2017). In the East Alborz (at longitudes of * 58�E),south of the Kopet Dagh, (Mattei et al. 2017) measured

paleomagnetic declinations of 22.5� in the Miocene Upper

Red Formation, resulting in a clockwise rotation of

18.4 ± 7.9� since the Miocene. Data from the Kopet Dagh

indicates a clockwise rotation of 19.4� and 14.3� since the

Cretaceous and Paleocene, respectively (Mattei et al.

2019). Their data match well vertical-axis rotation values

for only reverse direction (Fig. 12c) since the Paleocene

calculated here (21�). Furthermore, Mattei et al. (2017)

calculated a counter-clockwise rotation of 21.1 ± 11.6� atlongitudes between 52 and 54�E (Fig. 1). These published

paleomagnetic rotations suggest that the Kopet Dagh

underwent oroclinal bending. Earlier data by Bazhenov

(1987) and paleomagnetic results taken as a whole in this

study do not reveal any rotation along the northern

boundary of the Kopet Dagh mountains. This might indi-

cate that the Kopet Dagh Basin closed in a ‘‘scissor tec-

tonic’’ fashion (Zwaan and Schreurs 2017), where the

southern margin rotated clockwise and the norther one

remained stable. Thus, clockwise rotations would be

expected closer to the tectonic boundary with the East

Alborz and especially towards the southeast of the Kopet

Dagh. Therefore, further paleomagnetic direction mea-

surements need to be conducted in this area to test whether

the Kopet Dagh potentially represents a secondary orocline

resulting from superimposed wrenching (Model E from

Fig. 17 of Weil and Yonkee 2009).

6.3 Local folding kinematics

Although the reverse ChRM mean direction indicates 21�of regional clockwise rotation potenially resulting from

oroclinal bending, individual paleomagnetic mean direc-

tions of every site (Fig. 11) around the Kalat syncline show

a rotational pattern related to the syncline structure

(Fig. 15; red arrows). Although partially overprinted or

overlapping, this pattern may represent local rotations post-

dating major vertical-axis rotation as folding in the Kopet

Dagh happened earliest in the Oligocene (Berberian and

King 1981; Golonka 2004; Hollingsworth et al. 2010) and

maybe even in late Miocene times (Lyberis and Manby

1999). Furthermore, late Miocene growth strata along the

northern boundary of the East Kopet Dagh support this

interpretation of a late deformation stage. The site-mean

paleomagnetic directions describe a local rotation at each

site so that the normal line to the direction points towards

the syncline center, i.e. clockwise rotation in the northwest

and southeast and anti-clockwise rotation in the northeast

and southwest parts of the syncline (Fig. 15). Directions of

shortening inferred from the AMS ellipsoid orientation at

every site (Fig. 13) show a similar picture (Fig. 15; blue

arrows). The overall direction of shortening calculated as

an average of all site AMS ellipsoid measurements is ori-

ented towards 32�, which is perpendicular to the overall

strike of the Kopet Dagh (* 120�). Like paleomagnetic

directions, AMS ellipsoids are rotated clockwise in the

northwest and southeast and anti-clockwise in the northeast

and southwest, relatively to the overall mean orientation

(Fig. 15).

To test whether this observed pattern of rotation and

principle strain/stress orientations can be reproduced and

explained by 3D folding dynamics, paleomagnetic data are

compared to results of the numerical experiment of multi-

layer deformation (Fig. 14). Viscous flow of the relatively

weak material in between two stronger layers is charac-

terized by vertical-axis rotational strain rates depending on

the vertical and horizontal position (Fig. 14b). Rotational

flow close to the lower strong layer reproduces the rotation

pattern deduced from paleomagnetic directions (Fig. 15;

red arrows). Orientations of numerical principle stress

orientations close to the lower strong layer are comparable

to the shortening directions calculated from AMS ellip-

soids around the Kalat syncline (Fig. 15; blue arrows).

AMS data has been demonstrated to reflect the orientation

of the strain tensor (Borradaile and Henry 1997). However,

it has also been shown that the orientation of the strain

tensor is comparable to the one of the stress tensor when

inverstigating claystones and slates (Sagnotti et al. 1994).

The accordance of paleomagnetic results and numeri-

cally obtained viscous flow of the weak layer during three-

dimensional multi-layer folding in terms of local vertical-

axis rotation and principle stress direction helps under-

standing the structural evolution of the Kalat syncline.

Numerical results support the assumption that the Pale-

ocene continental red beds (Pestehleigh Formation) acted

as a weak decoupling horizon between the more rigid

carbonate-dominant horizons of the Kalat and Chehel

Kaman formations and therefore has been able to deform at

strain rates large enough to feature measurable local ver-

tical-axis rotations. Rotation strain-rate patterns within the

weak interlayer furthermore suggest that sample sites are

situated at the stratigraphic base of the Pestehleigh For-

mation (Fig. 14b). This is true except for site P04, which is

stratigraphically located higher up within the Pestehleigh

Formation.


The pattern of paleomagnetic directions and AMS

ellipsoid orientation around the Kalat syncline are a result

of three-dimensional folding and related deformation

within a relatively weak horizon. The doubly-plunging

shape of the syncline might be the result of the complex

underlying basement structure. Seismic sections cross-

cutting and parallel to the main strike of the Kopet Dagh,

i.e. along the shortest and longest syncline axis, respec-

tively, both illustrate the occurrence of basement faults

offsetting pre-Jurassic to Jurassic strata (Figs. 3, 4 and 5).

Major inherited normal faults strike parallel to the overall

Kopet Dagh (Amurskiy 1971) strike and act potentially as

reverse thrusts during tectonic inversion (Robert et al.

2014; Ruh and Verges 2018; Ruh 2019). These faults result

from Middle Jurassic rifting along the southern margin of

the Turan plate. Another set of rift structures, which strike

NNW–SSE, affected the whole Amu Darya Basin during

the Triassic (Brunet et al. 2017; Khain et al. 1991). We

argue that those two sets of normal faults resulted in

stratigraphic thickness variations parallel to and across the

Kopet Dagh Basin that enabled the growth of a doubly-

plunging syncline during their structural reactivation in the

Cenozoic. Forced three-dimensional folding then lead to

local vertical-axis rotation within the weak horizon, i.e. the

continental formation, resulting in the particular pattern

obtained by paleomagnetic measurements.

7 Conclusions

Paleomagnetic directions from continental red beds and

white carbonates around the Kalat syncline in the East

Kopet Dagh mountains, NE Iran, reveal that ChRM

directions may have been overprinted, are not fully isolated

or overlap with other components according to a reversals

test. The overall mean paleomagnetic direction of all

samples exhibits a declination of 12�, indicating no verti-

cal-axis rotation. On the whole, reverse directions suggest a

potential clockwise vertical-axis rotation of 21� along the

Kalat syncline, comparable to rotation in the South Kopet

Dagh. We therefore conclude that reverse directions rep-

resent clockwise vertical-axis rotation since the Paleocene

and that magnetic directions were partially overprinted

after major tectonic rotation. The overall mean direction of

shortening inferred from AMS ellipsoids is oriented per-

pendicular to the present strike of the northern boundary of

the Kopet Dagh, that is the Main Kopet Dagh Fault. Indi-

vidual paleomagnetic and AMS-related shortening

Fig. 15 Satellite image of the Kalat syncline. Location is outlined by

black box in Fig. 2. Red arrows: Paleomagnetic directions including

value for declination for sites P01–P05. Red shade: Error of the

paleomagnetic direction after Demarest (1983). Blue arrows: Direc-

tion perpendicular to the maximum axis of AMS ellipsoids for sites

P01–P06 indicating main shortening (site P07 is statistically not

relevant; see Table S1). Light blue arrows: AMS directions corrected

for bedding tilt. Bottom left: Legend and overall mean paleomagnetic

and AMS directions including values


directions show a characteristic pattern with respect to the

doubly-plunging syncline shape indicating potential local

rotation after magnetic overprint. A numerical model of

multi-layer forced folding has been conducted to test the

effects of complex folding kinematics on local vertical-axis

rotations. The experiment suggests that the clastic red beds

(Pestehleigh Formation) acted as a weak interlayer in

between more rigid carbonate horizons, which may explain

variations of post-overprint vertical-axis rotation for dif-

ferent locations around a growing doubly-plunging

syncline.

Acknowledgements The authors thank Bjarne Almqvist and an

anonymous reviewer for their constructive comments. This work has

been supported by Swiss National Science Foundation (Grant Number

2-77297-15). We thanks the National Iranian Oil Company who

organized field work and the Paleomagnetic Laboratory CCiTUB-

ICJTA CSIC where measurements were conducted.

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